U.S. Pat. No. 4,311,270 describes a centrifugal separator intended for dividing a solids containing liquid mixture into one liquid phase, which is substantially free of solids and has a relatively low viscosity, and one concentrate phase, which is rich in solids and has a relatively high viscosity. The centrifugal separator comprises a rotor, which is rotatable around a central rotational axis and which has an inlet for said mixture, an outlet for said liquid phase and an outlet for said concentrate phase. Characteristic of the centrifugal separator according to U.S. Pat. No. 4,311,270 is that its rotor has in its outlet for the concentrate phase a vortex device, which has the property that it can maintain the viscosity substantially constant of the concentrate phase flowing therethrough and out of the rotor. Thus, if the viscosity of the outflowing concentrate phase tends to increase, the vortex device automatically lets out a larger flow of the concentrate phase, and if the viscosity tends to decrease, a smaller flow of the concentrate phase is let out of the rotor. Thereby, the vortex device can be formed in a way such that a desired viscosity is always obtained of the concentrate phase separated in and leaving the rotor.
One embodiment of the centrifugal separator according to U.S. Pat. No. 4,311,270, which has been used in practice, is the one shown in FIG. 3 of said U.S. Pat. No. 4,311,270. This is used for instance for separation of yeast. In a centrifugal separator of this kind the radially outermost part of the rotor separation chamber, the so-called concentrate space, where separated yeast is accumulated during operation of the rotor, constantly communicates with a central chamber in the rotor, the so-called concentrate chamber, from where the yeast is discharged out of the rotor through a so-called paring member. At least one so-called concentrate tube connects the concentrate space with the concentrate chamber and at the radially innermost part of the concentrate tube a vortex device of the previously described kind is placed, so that the yeast may pass therethrough before entering the concentrate chamber.
A problem which has been noticed in connection with a centrifugal separator of this kind is that parts of the rotor and certain process conduits outside the rotor downstream thereof have not been sufficiently clean upon a conventionally performed cleaning of the centrifugal separator during rotation of the rotor. During a cleaning of this kind cleaning liquid is continuously supplied through the rotor inlet for mixture to be treated within the rotor, the cleaning liquid being discharged from the rotor through the ordinary rotor outlets for separated liquid phase and separated concentrate phase, respectively. The problem having been noticed is concerned with the flow paths for separated concentrate phase, which have not been cleaned to a desired extent neither within the rotor nor downstream thereof.
The reason for the problem is that said vortex device has the property--which is desirable during normal separation but not in connection with cleaning of the centrifugal separator--that it reduces a through flow of liquid if the viscosity of the liquid decreases. Since cleaning liquid has a substantially lower viscosity than the concentrate phase normally passing through the vortex device, the resulting flow of cleaning liquid in the flow paths for the concentrate phase becomes undesirably low, which leads to an insufficient cleaning of these flow paths. It has been noticed in some cases that the flow of cleaning liquid through the relevant flow paths has only been about 30% of the normal flow of concentrate phase during separation.
The problem here concerned does not arise, of course, only in connection with the embodiment of a centrifugal separator as discussed above. In any centrifugal separator, the rotor of which has a vortex device of one kind or another placed in the flow path for a liquid concentrate phase between a so-called concentrate space and a so-called concentrate chamber, the problem will arise, thus, also for instance in a rotor provided with a vortex device of the kind which can be seen from DE 36 13 335 C1 or DE 36 35 059 C1.